Laser communication system

ABSTRACT

An optical wireless transceiver for communicating broadband signals through free space includes an input, a regenerator, a splitter and a plurality of lasers in transmitter modules. A very fast (low f-number) optical receiver module includes a reflector aligned with an input aperture. A photodiode receives the signal from the reflector for subsequent demodulation. A background rejection filter is disposed between the reflector and the photodiode at the focal point of the mirror. The transceiver provides signal regeneration and switchable data rates. Connections are made to optical or electrical digital inputs and outputs bearing signals of various protocols. The plurality of lasers includes adjustable-beamwidth collimating lenses. An efficient high-current, high power laser driver capable of modulating a laser between 100 and 1500 mA at data rates greater than 10 Mbits/sec is provided. A highly efficient thermoelectric cooler operates to cool the laser diode, or other components requiring cooling.

PRIORITY

[0001] This is a divisional U.S. patent application Ser. No. 09/769,082,filed Jan. 24, 2001, the disclosure of which is incorporated herein byreference, to which priority under 35 U.S.C. § 120 is claimed.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The field of the present invention is laser communications.

[0004] 2. Background of the Invention

[0005] High quality video and audio signals and high bandwidth datasignals (called “broadband” signals) are becoming increasingly desirablein today's digital world. A significant challenge is getting highbandwidth communications to end users, or reaching the so-called “lastmile” market segment. Most U.S. metro centers are serviced by multipleproviders over SONET fiber optic rings, with fiber to certain majorbuildings. Many, if not most, buildings, are not on fiber rings,however, and laying fiber can be time consuming and prohibitivelyexpensive. In some instances, it may be practically impossible to obtainproperty rights-of-way to provide a high-bandwidth connection to thedesired location.

[0006] While wireless radio frequency (RF) systems can provide datarates of 155 Mbps, there is limited spectral bandwidth available,communication licenses are generally required, the possibility formutual interference exists, and the requisite equipment is expensive.Extending to higher data rates is difficult for RF frequencies with goodatmospheric propagation characteristics.

[0007] Atmospheric laser communication provides a potential alternativefor wireless point-to-point communications of high bandwidth signals.For instance, laser transceivers are capable of sending high bandwidthsignals through the atmosphere. However, commercially available lasersystems capable of transmitting high bandwidth signals across distanceslonger than a small city block are prohibitively large and extremelyexpensive. Moreover, several challenges must be overcome to facilitatehigh bandwidth laser communications over significant distances. Oneconsideration is ensuring reliable communications despite varyingatmospheric conditions. Since conditions such as fog in particular aredifficult for low power laser beams to penetrate, ensuring uninterruptedatmospheric laser communications requires the use of high power lasers.A second design consideration is preventing high power laser beams usedin an atmospheric laser system from causing eye or tissue damage ifreceived by people. At short wavelengths, non-eyesafe power levels canpermanently damage the eye before the victim becomes aware, because theretina has no pain sensors

[0008] Further complicating the use of atmospheric lasers is aphenomenon called scintillation that causes the random fading of signalstransmitted through the atmosphere. It is understood that the atmosphereis not homogeneous, in that the index of refraction of air is notconstant due to wind or turbulence. The transmission of a beam of lightthrough the atmosphere is subject to these variations in the index ofrefraction such that the beam may be momentarily deflected from astraight path. With such deflection, an observer of the beam perceivesthe source to be flickering. Such flickering is highly disruptive todata transmission. A solution may be found in aperture averaging, byincreasing the size of the apertures of the receiving unit. Theintensity of the source can, to a certain extent, mitigate losses intransmission where the sensitivity of the receiver is notcorrespondingly decreased. Often, however, physical and practicallimitations detract from such solutions.

[0009] To significantly overcome the effect of scintillation, spatialdiversity transmitters have been constructed which employ multiple diodelasers arranged to produce displaced parallel beams. As these beamsdiverge, they overlap one another. A receiver displaced from thetransmitter thus receives uncorrelated light at the receiver whenaligned with the beams. As it is unlikely that all beams will besimultaneously diverted, the receiver is able to receive uninterrupteddata from at least some of the plurality of transmitters. It has beenfound that the normalized standard deviation of the intensity at thereceiver is reduced by the square root of the number of transmittingelements when properly separated. Reference is made to W. M. Bruno, R.Mangual, & R. F. Zampolin, Diode Laser Spacial Diversity Transmitter,pp.187-194, SPIE vol. 1044, Optomechanical Design of Laser Transmittersand Receivers (1989), the disclosure of which is incorporated herein byreference.

[0010] One structural application of the very principles presented inthe foregoing publication is found in U.S. Pat. No. 5,777,768, thedisclosure of which is also incorporated herein by reference.Transceivers using spaced multiple laser transmitters are used fortwo-way communication.

[0011] Another example of laser transceivers used for communicationspurposes may be found in application Ser. No. 09/434,913, filed Nov. 5,1999, for a Portable Laser Transceiver, the disclosure of which isfurther incorporated herein by reference. The portable laser transceiverdisclosed therein is capable of transmitting near-broadcast qualityvideo, audio, and Ethernet signals.

[0012] For broadband fiber optic applications, a number ofpre-fabricated integrated circuits are available for driving lasers atdata rates of 1 gigabit per second (Gbps) or more. These laser driversare used to drive the now-common fiber optic networks. These integratedcircuits, however, are inadequate for high power lasers used inatmospheric laser communications, as they typically provide drivecurrent capability of only 50 to 75 mA. Such low drive current isinsufficient to overcome the effects of atmospheric scintillation atdistances beyond of approximately the length of a laboratory.

[0013] When a high power laser is used, one method that has been used toachieve the high drive current needed to overcome atmosphericscintillation effects is the use of a RF Bias-Tee. This method typicallyuses a 50 ohm bias tee, thus coupling the broadband signal into a 50 ohmload—typically consisting of a 47 ohm matching resistor in series with a3 ohm laser diode—to achieve a broadband match. The RF bias-teeapproach, however, is not practical for high drive currents because themajority of the output power is wasted in the matching resistor. Forexample, a 700 mA drive current typically results in 5.8 watts of powerdissipation in the 50 ohm bias tee.

[0014] A high current 4:1 broadband RF transformer may be used with abias-tee approach to double the output drive current and transform the50 ohm into a 12.5 ohm source, as seen by the load. However, thisalternative approach still requires a 9 ohm resistor to match the sourceto a 3 ohm laser diode. Thus, the majority of the drive power is stillwasted in the matching resistor. A transformer with a higher ratio couldtheoretically solve the lost power problem, but high ratio transformerscapable of handling currents in excess of 200 mA and having a broadbandresponse of up to 1 Ghz are not available.

[0015] U.S. Pat. No. 5,521,933 discloses a method of positioning thelaser diode remotely from the driver circuit to reduce the effect on thelaser diode of heat generated by the driver circuit. This method stilluses a matching resistor, located remotely from the laser diode, whichagain causes power loss in the output drive current.

[0016] Regardless of the driving frequency, when driving a laser diodeat high power, it is frequently desirable to use a thermoelectric cooler(TEC) to maintain the temperature stability of the laser diode. In manyapplications, the temperature stability of the laser diode may beimportant to maintain the output signal of the laser diode within aspecified set of parameters. The cooling function of a TEC is controlledby a TEC controller circuit. Typical implementations of TEC controllersare either pulse-width-modulated (PWM) or proportional controllers. PWMcontrollers are undesirable for use in communication systems withsensitive receivers because PWM controllers tend to generate unwantednoise. Proportional controllers such as the proportionalintegral-differential (PID) type are therefore commonly used in suchcommunication systems.

[0017] PID controllers, however, tend to dissipate the most heat whenmaximum cooling at the laser diode is required. The heat dissipationoccurs because PID controllers function as a current source, having acompliance voltage that is significantly less than the supply voltage.For example, a PID controller operating off a 5V supply at its maximumrated current output typically has a useable compliance voltage of about3V. The difference between the supply voltage and the useable compliancevoltage tends to be dissipated in the controller as heat. Thus, in themost demanding cooling conditions such as hot weather, a PID controllertends to generate even more heat.

[0018] A need, therefore, exists for small and efficient, yet powerfullaser transceivers that are capable of transmitting and receiving highpower and high bandwidth signals across distances greater than a singlecity block. A need also exists for a means to efficiently cool highpowered laser transmitters used in such transceivers.

SUMMARY OF THE INVENTION

[0019] The present invention is directed to transceivers using laserlight as the carrier and methods for laser communication fortransmitting broadband signals. Transmitter and receiver modules arecontemplated. Use of a high power, high frequency laser driverfacilitates free space communication across distances of at least eightkilometers in favorable weather conditions, and at least approximatelytwo kilometers in foggy conditions according to a London, England fogenvironment with 99% availability.

[0020] In a first separate aspect of the invention, a high power, highfrequency laser driver includes a power amplifier with a low outputimpedance suited to drive a laser diode. The power amplifier is operatedas a voltage-controlled current driver for the laser diode. The laserdriver is capable of providing very high current modulation, at least100 mA, at high data rates, at least 10 Mbps, to a laser diode.

[0021] In a second separate aspect of the invention, a laser transceiverincludes an input, a regenerator, a splitter receiving signals from theregenerator, a plurality of high power and high data rate laser drivers,and a plurality of lasers transmitting high bandwidth signals of thesplitter. At least one digital signal may be transmitted.

[0022] In a third separate aspect of the invention, a transceiver havinga regenerator includes a clock and data recovery circuit having multipleswitchable digital data rates. This switching may be performed bysoftware.

[0023] In a fourth separate aspect of the invention, a laser transceiverincludes a fast reflector characterized by a low f-number, a long wavepass background rejection filter adjacent to the focal point of a thereflector, and a photodiode having a predictable responsivity roll-offat a wavelength above the long wave pass frequency. The resultingcombination effectively operates like a bandpass filter.

[0024] In a fifth separate aspect of the invention, a high power laserdiode is stabilized against temperature fluctuations with an efficientthermoelectric cooling system. Aided by a temperature sensor, athermoelectric cooler in thermal communication with a laser diode iscoupled with a power amplifier that is operated as a controlled currentsource to supply current to the thermoelectric cooler at near-perfectefficiency when maximum cooling is required.

[0025] In a sixth separate aspect of the invention, a stand-alone radiofrequency transceiver is operated with at least one laser transceiver inoverflow mode. A router is used to monitor and distribute incoming datasignals between the laser and RF transceivers to promote enhanced totalsystem bandwidth capability and low switching latency.

[0026] In a seventh separate aspect of the invention, a lasertransceiver is enclosed within a protective enclosure and includes anenvironmental control system to maintain appropriate conditions for thetransceiver to operate.

[0027] In a eighth separate aspect of the invention, a laser diode isthermally coupled with a thermoelectric cooler and then enclosed withina housing having integral heat sinks to draw heat away from the diode byway of the thermoelectric cooler.

[0028] In an ninth separate aspect of the invention, a transceiver has aplurality of laser diodes, each of which is coupled with a lens forreceiving and collimating the laser output into a beam, with the lensessubject to beamwidth adjustment. Beamwidth adjustment between 0.3 andgreater than 3.5 mrad is provided.

[0029] In a tenth separate aspect of the invention, a laser transceiverthat is independent of particular digital protocol and is switchablebetween different data rates is provided. The transceiver may operate ondifferent physical layers including STS-3, STS-12, OC-3, and OC-12, andsupport various digital signal protocols including TCP/IP, IPX, FastEthernet, SONET, and ATM.

[0030] In an eleventh aspect of the invention, any of the foregoingaspects are contemplated in combination for additional advantage.

[0031] Accordingly, it is an object of the present invention to providean improved transceiver using laser communications, and an improvedlaser driver. Other and further objects and advantages will appearhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In the figures, wherein like numbers reflect elements havingsimilar function:

[0033]FIG. 1 is a schematic drawing of a laser communication system;

[0034]FIG. 2 is a front view of a transceiver according to a firstembodiment;

[0035]FIG. 3 is a side view of the transceiver in FIG. 2;

[0036]FIG. 4 is a schematic diagram of a transceiver and associateddevices according to a second embodiment;

[0037]FIG. 5 is a side sectional view of a transceiver and associatedhousing according to a third embodiment;

[0038]FIG. 6A is a front perspective view of a front portion of thehousing depicted in FIG. 5;

[0039]FIG. 6B is a rear perspective view of a front portion of thehousing depicted in FIG. 5;

[0040]FIG. 7A is a front perspective view of a rear portion of thehousing depicted in FIG. 5;

[0041]FIG. 7B is a front perspective view of a rear portion of thehousing depicted in FIG. 5;

[0042]FIG. 8 is a schematic diagram of a transceiver according to afourth embodiment;

[0043]FIG. 9 is a schematic diagram of a laser driver that may be usedwith the transceivers of FIG. 5 or 8;

[0044]FIG. 9A is a first magnified portion of the schematic diagramdepicted in FIG. 9;

[0045]FIG. 9B is a second magnified portion of the schematic diagramdepicted in FIG. 9;

[0046]FIG. 9C is a chart showing the values of components in theschematic diagrams depicted in FIGS. 9A and 9B;

[0047]FIG. 10 is a schematic diagram of a complementary circuit for thelaser driver of FIG. 9;

[0048]FIG. 10A is a first magnified portion of the schematic diagramdepicted in FIG. 10;

[0049]FIG. 10B is a second magnified portion of the schematic diagramdepicted in FIG. 10;

[0050]FIG. 10C is a third magnified portion of the schematic diagramdepicted in FIG. 10; and

[0051]FIG. 10D a chart showing the values of components in the schematicdiagrams depicted in FIGS. 10A-C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0052] Turning in detail to the drawings, FIG. 1 illustrates a lasercommunication system 10 comprising a first laser transceiver 12 and asecond laser transceiver 14, each transceiver 12, 14 having a digitalsignal input 20 and digital signal output 22 that carry thecommunication signals being transmitted and received. The signal input20 and the signal output 22 may carry either electrical or opticalsignals of various types. Compatible electrical signals may be carriedon electrical cables such as Category 5 cable with RJ 45 connectors, forexample, and the signals may conform to protocols such as TCP/IP, IPX,Fast Ethernet, or others known in the art, operating on physical layerssuch as STS-3, STS-12, or others known in the art. Compatible opticalsignals may be carried on fiber optic cables, and the signals mayconform to protocols such as SONET, ATM, or others known in the art,operating on physical layers such as OC-3, OC-12, or others known in theart.

[0053] Electronics 16, 18 within each transceiver 12, 14 use the digitalsignal input 20 to drive the outgoing laser signals and convert theincoming laser signals into appropriate digital signal output 22.Preferably, the laser communication system 10 is capable of transmittingand receiving laser signals a bandwidth of at least 155 Megabits persecond over distances of at least eight kilometers in favorable weatherconditions, and at least approximately two kilometers in foggyconditions according to a London, England fog environment with 99%availability. More preferably, the system is capable of transmissionacross like distances and conditions at a bandwidth of at least 622Megabits per second.

[0054] Preferably, each laser has a nominal output wavelength of 1.5 μm.One of the principal advantages of 1.5 μm wavelength is that are eyesafeintensity levels are approximately 50 times greater than at 0.8 μm. Thisis because for near-infrared wavelengths longer than 1.4 μm, the lightis absorbed by the lens and cornea, and is not focused onto the retina.For 1.5 μm, the eyesafe power levels are the same as levels for heatingof the skin, as it is the same effect. Also, at short wavelengths,non-eyesafe power levels can permanently damage the eye before thevictim becomes aware, because the retina has no pain sensors. At 1.5 μm,if non-eyesafe power levels are encountered, the sensation of heat (oreven pain) can be felt on the surface of the eyeball, and the naturalblink reflex is induced.

[0055] The use of 1.5 μm wavelength has other benefits. Since theeyesafe intensity level at 1.5 μm is 50 times greater than at 0.8 μm,and since the intensity is inversely proportion to the square of theaperture diameter, this means that for a given power level, the diameterof the transmitting aperture can be seven times smaller at 1.5 μm than0.8 μm. This facilitates the use of multiple transmitting apertures forscintillation reduction. Additionally, atmospheric attenuation isreduced at 1.5 μm relative to shorter wavelengths. The 1.5 μm wavelengthregion also corresponds to the low loss region for modern fiber optics.

[0056]FIGS. 2 and 3 illustrate one embodiment of a transceiver 50directed to laser communication. FIG. 2 shows a front view of thetransceiver 50 having a front plate 52 with a large centrally locatedreceiving aperture 54 and four laser apertures 56 spaced about thereceiving aperture 54. The diameter of the receiving aperture 54 isapproximately twenty (20) centimeters. A narrow support beam 58 crossesthe receiving aperture 54 and supports a photodiode 60, a backgroundfilter 61, and, if needed, one or more field correcting lenses 63, allof which are centrally located in relation to the receiving aperture 54.

[0057] High power laser transmitters 62 are disposed within each of thelaser apertures 56 such that the laser transmitters 62 emit beams thatare substantially parallel to each other. Each laser diode 62 respondsto an amplified laser data signal to generate an intensity modulatedlight signal. Each laser 56 preferably produces an average power of atleast eighty (80) milliwatts and has a nominal wavelength of 1,550nanometers (1.5 μm), within the eyesafe region. As the laser diode 62tends to emit a wide angle signal of about 300 cone angle, a lens 55receives the laser output. The lens design is flexible in setting thebeamwidth; that is, the beamwidth may be adjusted during production byadjusting the spacing between the lens 55 and the laser diode 62. Thisadjustment may be made by way of a threaded connection, or, morepreferably, by using a vacuum chuck and micrometer positioner toposition the lens. Once the desired lens position is reached, the lens55 may be epoxied in place. After the epoxy is cured, the vacuum isreleased, and the micrometer positioner is removed. The range ofbeamwidth (i.e. beam divergence angle) adjustment provided by the lens55 ranges from 0.3 mrad to greater than 3.5 mrad. If the laser diode 54has a spatially elliptical output, a circularizing optic (not shown),which may be a cylindrical lens or prism pair, may be added. As notedbefore, the lasers 54 are all aligned such that the collimated beams areparallel.

[0058] A sighting scope 64 may be located along an outer portion of thefront plate 52 for aiming the transceiver 50 at a second transceiver(such as the transceiver 14 shown in FIG. 1). As the sighting scope 64is intended for initial alignment of a transceiver pair (e.g., the lasertransceivers 12,14 shown in FIG. 1), the sighting scope 64 may beremoved after this alignment is attained. Alternatively, a CCD (chargecoupled device) sensor 65 may be added to a transceiver 50 and used tofacilitate alignment of a transceiver pair. Connection of the CCD 65 (orCCD 256 in FIG. 8) with a controller 250 is illustrated schematically inFIG. 8. When the CCD 65, 256 is coupled with a controller 250, a framegrabber 257 extension to the controller 250 may be used to captureimages of what is within the line of sight of the transceiver 12. Suchimages may be transmitted or downloaded from the controller 250 to aremote location, with an external interface operating a protocol such asSNMP, Ethernet, or another protocol known in the art.

[0059] Referring now to FIG. 3, the front plate 52 is connected to arear plate 66 using support rods 68. The rear plate 66 supports afocusing reflector 70, which is centrally aligned with the receivingaperture 54 such that the photodiode 60 is positioned at the focal pointof the focusing reflector 70. The focusing reflector 70 is approximatelyequal in diameter to the aperture 20 and facing in a parallel directionwith the lasers 62.

[0060] The receiver portion of the transceiver 50 includes a receivingaperture 54, a focusing reflector 70, a photodiode 60, a backgroundrejection filter 61, and electronics 72. The focusing reflector 70focuses the incoming optical beam onto a photodiode receiver 60. Thefast design enables the large aperture 54 for collection efficiency, yetprovides a very short focal length—both for compactness, and to achievethe largest field-of-view for a given detector diameter. The preferredf-number of the reflector 70 is approximately 0.67. As the f-number ofthe focusing reflector 70 decreases, off axis optical aberrationsincrease.

[0061] Mangin mirror or parabolic reflector approaches may be used forthe focusing reflector 70 according to separate embodiments. Acatadioptric design, the Mangin mirror is a negative meniscus lens witha mirrored rear surface which combines a compact overall design with ashort focal length. The presence of both reflective and refractiveelements in a Mangin mirror provides sufficient degrees of freedom tokeep the amount of optical aberration within acceptable limits over theentire optical field of view, such that no additional optical correctorlens is necessary.

[0062] A parabolic reflector may be used for the focusing reflector 70according to an alternative embodiment. A parabolic reflector has asingle reflective surface, and requires one or more field correctivelenses to sufficiently correct aberrations. The parabolic reflector maytherefore comprise one or more optical elements 63 to control the amountof aberration near the edges of the field of view. In placing thecorrector lenses 63 near the focal plane, the corrector lenses 63 may bemounted in the same mechanical assembly that holds the detector 60.Multiple corrector lenses may be placed thusly to provide a higherdegree of aberration control.

[0063] In a third reflector embodiment, the focusing reflector 70comprises a mirror having a more general conic or aspheric opticalsurface and one or more corrector lenses (not shown). A conic oraspheric mirror provides a limited amount of aberration control, thusreducing the number of corrector lenses needed to properly focus theincoming signals.

[0064] To reduce interference and background noise from ambient lightand other sources, the optical path of the receiver advantageouslyincludes a flat or hemispherical background rejection filter 61. Thefilter 61 may be comprised of alternating layers of dielectric materialdeposited on a glass or crystalline substrate. This construction resultsin a peak transmission at band center of over 65%. The filter 61 may belocated at the receiving aperture 54. However, such a configurationrequires a large and costly filter, adding weight and size to theassembly 50. Preferably the filter 61 is located between the focusingreflector 70 and the photodiode 60. A bandpass filter nominally centeredat 1500 nanometers may be used. However, if the bandpass filter is aflat multilayer dielectric filter, when receiving light rays from bothsmall and large angles of incidence with respect to normal characterizedby a low f-number optical system, then the passband is shifted for thedifferent angles of incidence. To correct this, then, a much largerpassband for a flat filter is to be used (about a 400 nanometer filter),or a hemispherical filter may be used since all of the incident rays arenormal to the filter surface. A hemispherical filter centered at thenominal laser wavelength of 1,550 nanometers and having a passband widthof 100 nanometers or less may be used.

[0065] A bandpass filter inherently requires the use of multilayerdielectric technology. A long wave pass approach may be used inconjunction with a detector 60 having predictable responsivity roll-offto create an effective bandpass filter. In one embodiment, the detectorresponsivity rolls off significantly between 1500 and 1700 nm, and iseffectively zero beyond 1700 nm. Coupling this detector 60 with a longwave pass filter transmitting 1500 nm and above wavelength, theresulting combination provides an effective bandpass filter. A long wavepass filter, as used in conjunction with a detector as described above,may be dielectric, and flat or hemispherical in shape. As an alternativeto a long wavepass multi-layer dielectric filter, the filter 61 may bean absorptive filter.

[0066] If a hemispherical filter 61 is used, its center of curvature islocated near the focal point of the reflector 70. Using a standard TO-8size detector package results in a hemispheric filter dome having anouter diameter of approximately twenty-two (22) millimeters and athickness of 2.5 millimeters. In an embodiment utilizing one or morecorrector lenses 63, the corrector lenses 63 may be used to create anafocal region in front of the photodiode 60 in which a filter 61 havinga flat shape would be placed.

[0067] The transceiver may also include a sealed protective enclosure 74as shown in FIG. 3. The protective enclosure 74 protects the transceiverfrom weather conditions and provides an enclosed environment in whichthe transceiver may operate. At the front cover of the protectiveenclosure 74 is an acrylic filter 76 covering the large aperture 54. Theacrylic filter 76 is transparent to the operational wavelength of thelaser transmitters 62, but limits transmission of visible light toprevent introduction of noise and heat from this light. Further, a straylight baffle 75 is preferably placed between the transceiver 12 and theacrylic filter 76 to reduce interference from stray light, andelectromagnetic and RF sources. The baffle 75 is preferably a honeycombof thin aluminum, approximately three inches in thickness, with aface-to-face hex cell size of approximately {fraction (11/32)} inch.

[0068]FIG. 4 is a schematic representation of a laser transceiver 100according to another embodiment. The transceiver 100 includes a combinedlaser transmitter/laser driver module 110, a receiver module 120, andassociated electronics 130. Starting with the receiver module 120, anincident laser beam is reflected by a reflector 122 and focused througha background rejection filter 123 onto a photodiode 124 located at thefocal point of the focusing reflector 122. From the photodiode 124, thesignal is carried to a preamplifier 125 preferably contained within thelaser receiver module 120, and then sent to signal conditioningelectronics 131. After conditioning, the signal is provided to an outputsignal interface 132, which may include a fiber transmitter fortransmitting an optical signal to a fiber optic cable. Electricalsignals may alternatively be output, such as via a Category 5 cable. Aswitching device 140, which may include a switch or router portion toenable transmission of various electrical or optical signals (includingTCP/IP, IPX, Fast Ethernet, SONET, ATM, or other signal types, onvarious physical layers such as STS-3, STS-12, OC-3, or OC-12) receivesthe output signal.

[0069] In the illustrated embodiment, the switching device 140 iscoupled to a computer 142 having a network interface card 143. Thepersonal computer 142 may further have an audio/video interface card 144for receiving audio and video signals. The switching device may beconnected with various electrical or optical input types, such asCategory 5 cable using RJ-45 connectors, or fiber optic cable. Anoptional stand-alone RF (radio frequency) backup transceiver 146 mayfurther connect to the switching device 140.

[0070] On the input/transmitter side, high bandwidth digital inputsignals are provided to the transceiver 100 via a switching device 140.A high bandwidth signal is conventionally considered to be a signal of10 Mbps or greater. One skilled in the art will recognize that any highbandwidth signal may be used as input to the transceiver 100, so long asappropriate electronics are provided, when necessary, to convert thehigh bandwidth input signal into a high bandwidth digital signal. Inputsignals may be provided through the switching device 140 to thetransceiver 100 by way of an input signal interface 134, which in oneembodiment may include a fiber optic receiver for receiving digitalsignals from a fiber optic cable. The signal is then provided to signalconditioning electronics 135, and thereafter provided to a splitter 136.The splitter 136 splits the signal into four identical signals atone-quarter of the power. Each signal emerging from the splitter 136 isprovided to a laser driver circuit 111 and ultimately provided to alaser diode 115. The laser driver circuit 111 includes a DC bias circuit112 and a high power, high bandwidth power amplifier 114 for driving thelaser diode 115. A high power laser driver is conventionally consideredto be a laser driver operating at a nominal level of 100 mA or greater.The high power, high bandwidth laser driver circuit is discussed indetail hereinafter, in connection with FIGS. 9-10C. The output of thelaser diode 115 is provided to a lens 116, and may be monitored with afiber optic segment 117 and fed to a photodiode 118 for monitoring theoutput of the laser diode 115. A thermoelectric cooler 119 and itsassociated controller 113 are provided to control the temperature of thelaser diode 115. The controller 113 is coupled with the thermoelectriccooler 119, which is in thermal communication with the laser diode 115.The controller 113 and the thermoelectric cooler 119 act in conjunctionwith a temperature sensor (not shown) to stabilize the temperature, andthus the output power, of the laser diode 115. The optimum operationaltemperature of each laser diode 115 depends on the specifications forthe particular laser diode used, as provided by the laser diodemanufacturer.

[0071] Preferably, output of the laser diode 115 is monitored. Onemethod of monitoring the laser diode 115 is by monitoring its inputcurrent, but this method is indirect. A better method is to monitor thetransmitted light signal from the laser diode 115. One method ofmonitoring the light signal is by way of a fiber optic element 117 thatmay extend into the transmitted light signal from the laser diode 115.This fiber optic element 117 is coupled with a photodiode 118. Morepreferably, the laser diode 115 is provided with an integral monitorphotodiode (not shown), which eliminates the need for a separate fiberoptic element 117. The output of the photodiode may be coupled with themodulation signal amplifier 114 to use the sampled laser diode outputfor controlling the amplifier 114 and in turn the signal strength, thuspermitting operation in constant optical power mode. Thus, themodulation signal amplifier 114 may be made responsive to the output ofthe photodiode 118.

[0072]FIGS. 5, 6A, 6B, 7A, and 7B illustrate an alternative compact andhigh power laser transceiver 150 and its associated housing 151. Thehousing 151 is designed to both mount the operative components and serveas a protective enclosure suitable for outdoor use. Only three parts,namely, a front portion 152, a rear portion 153, and a snout 182, aredesigned to interconnect to form the housing 151 to fully enclose thetransceiver 150. Of these three parts, the optical payload mounts toonly one: the front portion 152. This construction promotes precisepositioning of the optical components, since in comparison to theembodiment of FIGS. 3-4, this approach limits the adverse effects ofadditive dimensional tolerances for multiple structural parts. The frontportion 152 includes prominent annular heat sinks 184 to dissipate heatfrom laser diodes (such as the laser diodes 115 shown in FIG. 4) andfrom laser drive circuits and associated electronics (such as shown inFIG. 4) contained within the housing. More specifically, an innersurface of each heat sink 184 is in thermal communication with at leastone thermoelectric cooler (such the pair of thermoelectric coolers shownin FIG. 8), which in transmit heat from the laser diodes (such as shownin FIGS. 4, 8) to the heat sinks 184. Preferably, an annularthermoelectric cooler disposed around the laser diode 115 is used.

[0073] The front portion 152 is preferably cast from a lightweightaluminum alloy, but may alternatively be manufactured by varioustechniques from a variety of suitably durable, strong, and thermallyconductive materials, including steel. The housing 151 is designed tomate with the optical payload at four flat interior payload matingsurfaces 179 adjacent to the laser apertures 156, preferably by way ofscrews and tapped holes. The payload mating surfaces 179 further serveas a primary contact surface for transferring heat from laser diodes tothe heat sinks 184 by way of thermoelectric coolers. The cast housing151 is preferably machined along the payload mating surfaces 179 toensure that all four payload mating surfaces 179 are commonly flat. Thefront portion 152 is preferably painted to reduce effects of corrosionwhen subjected to outdoor use. The rear portion 153 does not contain anydedicated heat sinks 184, and therefore does not necessarily need to befabricated from a thermally conductive material. The rear portion 153may be manufactured from a plastic suitable for outdoor use, but couldalso be manufactured using other techniques and other durable materialsincluding metals.

[0074] The front portion 152 and rear portion 153 mate along a commonsurface 155 to form the housing 151 that encloses a reflector 170, fourlaser transceivers (not shown), a photodiode 160, a background rejectionfilter 161, one or more field corrector lenses 163, and variouselectronics 172. The photodiode 160, background rejection filter 161,and one or more field corrector lenses 163 are disposed at the focalpoint of the reflector 170, and supported by way of a narrow support rod158 placed across a primary aperture 154. Adjacent and connected to thefront portion 152 of the housing 151 is a snout 182 across the primaryaperture 154. The snout 182 preferably includes a honeycomb baffle 175and acrylic filter 176 for the same reasons discussed above in anothertransceiver embodiment. The snout further includes a protruding hood 183that further reduces interference from incident light and provides ameasure of weather protection for the front cover 177.

[0075] Wired (electrical or optical) data and power signals to and fromthe transceiver 150 are carried to the transceiver 150 by a conduit 186.The transceiver 150 may be supported from below by way of a mountingelement 187. The mounting element 187 preferably the angular position ofthe transceiver 150 to be adjusted to facilitate aiming the transceiver150 at a similar transceiver located remotely.

[0076]FIG. 8 provides a schematic representation of a wideband lasertransceiver 200 according to a fourth embodiment. On the input side, thetransceiver 200 includes various input signal electronics 202, a laserdriver circuit 220, and a laser transmitter module 225 with associatedthermoelectric coolers 226, 227. On the output side, the transceiver 200includes a laser receiver module 229 and various output signalelectronics 230. In the illustrated embodiment, the transceiver 200further includes an associated controller 250 in communication with anenvironmental control system 252, at least one temperature sensor 254, aCCD sensor 256, and an interface for diagnostics, monitoring, networkmanagement, and/or control 258. The controller 250 may further include aframe grabber 257, may receive signal level information from thereceiver module 229, and may receive monitoring information from thelaser diode.

[0077] Turning to the input/transmitter side, a first input signal isprovided to an input signal interface 204. The input signal is awideband digital signal, either electrical or optical, according to avariety of signal types (including TCP/IP, Fast Ethernet, SONET, ATM, orother signal types known in the art, on various physical layers such asSTS-3, STS-12, OC-3, or OC-12). Various electrical or fiber optic cableconnectors may be received by the input signal interface 204 dependingon design requirements. Upon receipt of the digital input signals, thesesignals are provided to a regenerator 206. The regenerator 206preferably includes an input signal amplifier 207, either a limitingamplifier or automatic gain control, to provide gain, as necessary, andsmooth out variations in input signal amplitude. The regenerator 206further includes a first clock/data recovery circuit 210 to provideprotocol independence. The first clock and data recovery circuit 210includes at least one oscillator 212, such as a crystal, to regulate theclock frequencies. When more than one oscillator is provided, theintegrated circuit utilizes two phase locked loops to permit modulationas different frequencies, so as to accommodate more than one datatransmission protocol, such as TCP/IP, Fast Ethernet, SONET, and ATM.Thus, a single transceiver 200 is enabled to transmit and receive morethan one of the aforementioned protocols. Switching between clockfrequencies may be performed by manually substituting an existingoscillator 212 with an oscillator of a different frequency. Morepreferably, as shown in FIG. 8, multiple oscillators 212, 213 may bebuilt into the transceiver 200, and a switch 214 may accomplishswitching between the oscillators 212, 213. This switching may becontrolled manually, or, more preferably, by a controller 250, such asby using transistor-transistor logic. Either oscillator 212, 213provides a clock signal to the data recovery electronics 215.

[0078] Upon regeneration, the digital signal may be provided to anoptional multiplexer 216 to be combined with at least one additionalsignal from a second input signal source 203. The additional signal maybe of various types, such as T1, Ethernet, telemetry information,tracking information, or other signals known to those skilled in theart. Thereafter, the combined signal is provided to a splitter 218 thatdivides the digital signal input into four substantially equalcomponents comprising the same signal input at one-quarter of the power.The four laser data signals are transported to a plurality of highpower, high frequency laser driver circuits 220, each of which modulatesa laser diode contained within the laser transmitter module 225. Onlyone of the four laser drivers 220 is illustrated. The remaining threelaser drivers 220 and laser diodes (within the laser transmitter module225) are preferably identical to those depicted. The laser diodes withinthe transmitter modules 225 are displaced from one another, aligned, andfacing in parallel directions. As previously described, each diodepreferably has a collimating lens (not shown).

[0079] In addition to modulating the laser diode within the lasertransmitter module 225, the laser driver circuit 220 controlsthermoelectric coolers 226, 227 associated with the laser diode tostabilize the temperature, and thus the output power, of the laserdiode. Further details regarding control of thermoelectric coolerassociated with the transceiver 200 will be discussed hereinafter.

[0080] Turning to the output/receiver side, an incident laser beam isreceived by the laser receiving module 229 and converted to apreamplified electrical signal. The signal is provided to output signalelectronics 230, which includes several elements. First the signal isprovided to a second regenerator 232. The regenerator 232 preferablyincludes a signal amplifier 234, either a limiting amplifier or anautomatic gain control, to provide gain, as necessary, and smooth outvariations in signal amplitude. The second regenerator 232 preferablyfurther includes a second clock/data recovery unit 236. The second clockand data recovery unit 236 preferably also comprises a switchable dualoscillator such as the dual oscillator 212, 213 shown in connection withthe first clock and date recovery unit 210, and operates in a similarmanner. Upon regeneration, the resulting digital electronic signal maybe provided to an optional de-multiplexer 238 to segregate at least oneadditional output signal 205. Following de-multiplexing, the digitalsignal remaining in the output signal electronics 230 is passed to anoutput signal interface 240. An output signal interface 240 converts theelectronic signal into an appropriate output signal, such as anelectronic or optical signal. Either electrical or optical interfacesmay be provided, subject to the inclusion of appropriate hardware aswould be apparent to one skilled in the art. In one embodiment, theoutput signal interface 240 includes a fiber transmitter that convertsthe electronic signal to an optical signal, and further includes a fiberoptic connector for connecting a fiber optic cable. The primary outputsignal is a wideband digital signal, and may be in accordance withvarious protocols known in the art, including TCP/IP, IPX, FastEthernet, SONET, or ATM, and may operate on various physical layersknown in the art, including STS-3, STS-12, OC-3, or OC-12. Althoughdepicted separately, the output signal interface 240 and input signalinterface 204 may be combined, and if the interfaces 204, 240 include afiber optic transmitter and a fiber optic receiver, then an integratedfiber optic transceiver may be used.

[0081] A controller 250, preferably a microprocessor-based digitalcontroller or computer, is advantageously provided, primarily to detectand monitor various aspects of the transceiver 200. This capability mayprovide both real-time and historical data logging for statusmonitoring, performance monitoring, network management, and/ortroubleshooting. When the transceiver 200 is first powered, the computer250 may sample the amount of current being supplied to the lasersthrough the laser drivers 220 to ensure that the lasers sufficient powerto transmit signals. During operation, the computer 250 monitors thetemperature levels of the laser diodes within the laser transmittermodules 225. If the temperature of any of the diodes exceeds the optimumoperational temperature as specified by the laser diode manufacturer,then the computer 250 may take one or more steps to alleviate theoverheating, such as alerting appropriate technical personnel, shuttingdown one or more of the laser transmitters 225, or any other appropriateactions. The computer 250 preferably also monitors the voltages suppliedto the photodiodes (not shown), the modulation signal to the laserdriver amplifier within the laser driver circuit 220, and the DC biascircuit (also part of the laser driver circuit 220). The measuredvoltages are preferably converted to a current value that is compared topre-set values in a look-up table, customized for the individual laserdiode, to further ensure the proper functioning of the transceiver 200.If the transmitted laser beam signal is monitored with a photodiode, theoutput of the photodiode may be coupled with the controller 250, toprovide laser performance information, or permit the lasers to beoperated at constant signal strength. The controller 250 may alsomonitor signal level from the laser receiver module 229. Furthermore,the controller 250 and may receive signals from a CCD sensor 256, and,utilizing a frame grabber extension 257, the controller 250 may captureimages of what is within the line of sight of the transceiver 200.

[0082] One interface that may be used by the controller 250 to performits functions is a RS-232 interface format utilizing SNMP (SimpleNetwork Monitoring Protocol), although other interface formats,including establishment of an Ethernet connection with the controller250, may alternatively be used. In a preferred embodiment, thecontroller 250 is a computer which is used primarily for diagnosticpurposes and may be polled remotely using a modem, over the Internetusing an (Ethernet) HTML browser, or other such communication means asare known in the art, thus permitting remote information monitoringand/or control. While the controller 250 is preferably amicroprocessor-based digital computer, a dedicated digital signalprocessor may alternatively be used.

[0083] Additionally, the controller 250 interfaces with environmentalcontrols 252 including additional temperature sensors 254, as may beprovided within an enclosure surrounding the transceiver 200, toregulate the temperature and humidity experienced by the transceiver 200and provide optimal environmental conditions for the transceiver 200 tooperate. Heat sinks, such as the heat sinks 184 provided in FIGS. 5-6Bmay also be added to the outside of a transmitter enclosure or housing151 to aid with environmental control.

[0084] Referring again to FIG. 4, a stand-alone radio frequency (RF)transceiver 146 is provided as a backup to the laser transceiver 50during inclement weather conditions, particularly fog. In one embodimentdirected to Fast Ethernet/OC-3 signals, the RF transceiver 146 operatesat a frequency of 2.4 GHz and is capable of up to 11 Megabits per second(Mbps) bandwidth. One of several different methods may be utilized toswitch between the laser transceiver 100 and the RF transceiver 146 whennecessary. First, a router 140 may be utilized to monitor the lasertransceiver 100 for repeated requests for retransmissions or packeterrors. When the requests for retransmissions or packet errors reach apredefined level, then the router 140 switches over to the RFtransceiver 146 for a predetermined amount of time. After suchpredetermined time, the router 140 switches back to the lasertransceiver 100 and again monitors the laser transceiver 100 forrequests for retransmissions or packet errors.

[0085] A second method that may be utilized to determine when to switchfrom the laser transceiver 100 to the backup RF transceiver 146 is bycoupling the controller 250 (as shown in FIG. 8) to the preamplifier ofthe laser receiver module 229 (as shown in FIGS. 4, 8) to monitor thestrength of the incoming signal. Using this method, once the incomingsignal drops below a predefined level, the controller 250 divides thesignal between the RF transceiver 146 and the laser transceiver 100. TheRF transceiver 146 is used to send the data signals while the controller250 continues to monitor the strength of the incoming signal from thelaser transceiver 100. Once the strength of the incoming signal from thelaser transceiver 100 returns to above the predefined level, thecomputer 250 switches back to using the laser transceiver 100.

[0086] A third method, called overflow mode, has the least latency ofthe methods. This method may be utilized to determine when to switchfrom the laser transceiver 100 to the backup RF transceiver 146 throughthe use of a router 140 to monitor and distribute the incoming datasignals between the two transceivers 100, 146. All signals are initiallyrouted through the laser transceiver 100 until the bandwidth of thelaser transceiver 100 is entirely in use. Excess data signals are thenrouted through the RF transceiver 146. As the bandwidth of the lasertransceiver 100 drops due to inclement weather, more data signals willbe routed to the RF transceiver 146. Conversely, as the weather beginsto clear and the laser transceiver 100 becomes more capable of carryingdata signals, less data signals will be routed to the RF transceiver146. This configuration has the further benefit of providing a highertotal system bandwidth capability with less switching latency.

[0087] A laser driver 300, such as for use as the laser driver 220depicted in FIG. 8, will now be described. Reference will be madegenerally hereinafter to FIGS. 9 IOC, which provide a laser driverschematic. The laser driver 300 essentially comprises a modulationsignal amplifier to amplify the laser data signals, and a DC biascircuit to ensure that a laser diode 301 operates in a relatively linearrange. The laser driver 300 is capable of providing very high currentmodulation, in the range of 100 mA to 1500 mA, at high data rates, inthe range of 10 Mbps to 622 Mbps, to the laser diode 301. The ability todrive the laser diode 301 at such high rates derives from a uniqueconfiguration involving a power amplifier. The power amplifier, which ispreferably a RF power FET, directly drives the low impedance laser diodeusing a low voltage power supply. A typical laser diode useful in thisapplication has a characteristic dynamic impedance of betweenapproximately 2 and 5 ohms. Preferably, the power supply voltage is lessthan 12 volts; more preferably, the power supply voltage isapproximately 5 volts. Higher supply voltages may also be used, such as12V, 15V, or 28V, but in using the higher supply voltages results ingreater power consumption and wasted thermal load. In applications wherethe power consumption and thermal load is less of a concern, the highersupply voltages may be used to attain improved gain and bandwidth.Preferably the RF power FET is capable of operating at a minimumfrequency of 1 MHz or less, and further preferably capable of operatingat a frequency of at least 100 kHz.

[0088] FIGS. 9-10C illustrate an embodiment of the laser driver 300 inwhich the output stage of the RF power amplifier Q3 is coupled with a 5Vsupply. The circuitry illustrated in FIGS. 9-9C comprises the primarylaser driver circuitry and is connected to the circuitry in FIGS. 10-10Cthrough connectors J1, J2, J3. The circuitry illustrated in FIGS. 9-9Cprimarily comprises the laser bias current circuitry and the TECcontroller circuitry. This particular embodiment of the laser driver 300is designed to accept a digital input signal having an amplitude of 175mV on a 50 ohm coaxial transmission line, with all the amplifiersoperating at least 2 dB below the −1 dB power output compression point.In addition, this embodiment comprises three stages of amplification. Inan alternative embodiment, two stages of amplification may be used.

[0089] As illustrated in FIGS. 9-9C, the input signal from the coaxialtransmission line is AC-coupled into the first amplifier stage Q1, whichmay comprise an ERA-2SM or similar op-amp. The signal then passes to adigital attenuator U2, which may comprise an RF2420 or similarattenuator. The digital attenuator U2 reduces the signal amplitude by aminimum of the 2 dB insertion loss and up to a maximum of 30 dB or more.The signal passes from the attenuator to the second stage amplifier Q2,which may comprise an ERA-6SM or similar op-amp. The digital attenuatorU2 may also be located after the second stage amplifier Q2. Followingthe second stage amplification, the signal is AC-coupled into the thirdstage power amplifier Q3, which may comprise an F2248 or D2202UK ordevice. In the embodiment shown, the power amplifier Q3 comprises abroadband RF power MOSFET. The power amplifier used should be one whichprovides the gain required as described herein and has at least a 1 GHzbandwidth. A silicon device may be preferred in certain instances over aGaAs device because the silicon device meets at least the minimumrequirements and typically provides a cost savings over GaAs devices.The laser diode 301, the sense resistors R11, R19, and the poweramplifier Q3 may alternatively be connected in series across the 5Vsupply voltage.

[0090] The first stage amplifier Q1 and the second stage amplifier Q2are broadband 50 ohm amplifier gain blocks. Alternatively, discrete RFtransistors may be used in the first and second amplifier stages. Inapplications where the input signal has a higher voltage level, or inapplications where less amplification is required to drive the laserdiode, the first and second amplifier stages may be replaced by a singleamplifier which provides the required gain for the input to the poweramplifier Q3. In an alternative embodiment, the first stage amplifier Q1may be a limiting amplifier that hard-limits a digital input signal to afixed output lever, regardless of the input signal amplitude. Typically,this limit would be a PECL (positive emitter control logic) output levelof 700 mV.

[0091] In the embodiment depicted in FIGS. 9-9C, the signal input to thesecond stage amplifier Q2 is at a higher voltage level than the signalinput to the first stage amplifier Q1. Therefore, in order to achievethe desired overall amplification, the second stage amplifier requires ahigher supply voltage, preferably greater than 7V. Alternatively, if theoverall power consumption needs to be reduced and if the reduced overallamplification is sufficient for a particular application, the secondstage amplifier Q2 may be operated from the same 5V power supply as thefirst stage amplifier Q1 and the power amplifier Q3. Supply voltagesgreater than 5V may also be utilized, however, using higher voltagesresults in proportionally greater power consumption and wasted thermalload. In the embodiment depicted, the 5V supply voltage has been chosento decrease power consumption and the thermal load.

[0092] Each amplifier stage is thermally stabilized using accepteddesign techniques to compensate for current and gain changes as afunction of temperature. Resistors R2, R8, and R20 are thereforeprovided to the first, second, and third stage amplifiers respectively.A DC bias voltage is provided to each amplifier stage through one ormore RF chokes in the form of inductors L4, L9, L7, L10, L2, L3, and L8.The RF chokes present a high impedance to any power supply noise thatmay range from 100 kHz to 1 GHz that could otherwise contaminate theamplified signal. Additionally, an AC-coupling capacitor C11 is providedto prevent the laser bias current from interfering with the DC operatingsupply voltage at the output of the power amplifier Q3.

[0093] The amplitude of the input signal to the power amplifier Q3 isadjustable, thus enabling the laser driver 300 to accommodate laserdiodes with differing drive current requirements. Adjustable feedback istherefore provided at the second stage amplifier Q2 by linking theoutput of the second amplifier Q2 to its input through and adjustableresistor R21. Such adjustable feedback could also be provided across thefirst stage amplifier Q1 or the power amplifier Q3. Alternatively,adjustable feedback could be achieved through the use of a voltagecontrolled variable gain amplifier at either the first or second stageof amplification, wherein a potentiometer would be used to adjust thegain of the amplifier.

[0094] The output drive current of the power amplifier Q3 to the laserdiode is adjusted to the nominal operating point of the laser diode byadjusting the gate voltage at the input of the power amplifier Q3. Thegate voltage is controlled by a first integrated circuit U3 and apotentiometer R6. The first integrated circuit U3 is preferably a CD4051BCM or similar device. The gate voltage is derived from a supply voltageregulated by a zener diode D1 to achieve a highly stable bias voltageregardless of power supply voltage fluctuations. The power amplifierstage Q3 is AC-coupled with the gate voltage such that the voltagewaveform at the input results in a linear modulation of the laser drivecurrent at the output. Therefore, the low impedance laser diode 301,being essentially a current-controlled device that linearly convertsinput current to output optical power, is driven by the voltagecontrolled power amplifier Q3 which resides in the low impedance poweramplifier stage of the laser driver 300. The input and the output of thepower amplifier Q3 are AC-coupled, and the output to the laser diode 301is provided with an appropriate dc bias current such that the outputmodulation of the power amplifier Q3 causes the laser drive current toswing from nearly off to the desired output power with an optical outputpower extinction ratio of at least 10:1. In designing the bias circuit,consideration may be given to selecting minimum, maximum, and averagepower levels for the laser diode 301, as bias current causes the laserto operate at a selected average power level, and the wideband signalmodulation will cause the laser output to vary between the power levelextremes.

[0095] The laser driver 300 illustrated in FIGS. 9-9C also comprisescircuitry for adaptive digital control of the output power of the laserdriver 300. Such control over the output power may be desirable so thatthe output power can be reduced when high output power is not needed.Reducing the output power may enhance the overall life span of the laserdiode by reducing operating stress levels during periods of operation.Control over the output power also enables the laser driver 300 to beused under a wider variety of circumstances. For example, the outputpower can be greatly reduced for communication links over shortdistances, thus avoiding saturating the receivers, and can be increasedto maximum or near maximum levels in order to overcome inclement weatherconditions.

[0096] Digital control over the output power is implemented bysimultaneous control over three laser driver functions. First, thedigital attenuator comprises a digitally controllable RF attenuator suchas RF Micro Devices RF2420. A 3-bit control signal data0, data1, data2is used to select the attenuation of the modulation signal. Typically, a1 to 3 dB insertion loss is selected, however, a 30 dB or more insertionloss may be selected as desired by the user.

[0097] Second, the laser bias current is also to be appropriatelyreduced to maintain the desired extinction ration of at least 10:1. Thelaser bias current is adjusted using multiplexer U7, which is configuredso that the same 3-bit control signal data0, data1, data2, selects arequired discrete resistor value using resistors R33, R22, R29, R30,R31, and R32. The discrete resistor value is used to realize anappropriate laser bias current of up to 500 mA from the output of thelaser bias current FET U4. This FET U4 preferably comprises an NDT 456 Por similar device. Potentiometer R18 is provided as a vernier control toset the nominal bias current required by the particular laser diodebeing used. The bias current output from FET U4 is applied to the laserdiode through a broadband choke network comprising inductors L5, L6 toensure any RF noise that might get picked up by the bias currentcircuitry is blocked and does not contaminate the laser modulationcurrent.

[0098] The third function controlled by the 3-bit control signal is thegate bias voltage for the power amplifier Q3. This adjustable gate biasvoltage is accomplished using multiplexor U3 configured so that the3-bit signal data0, data1, data2 selects a required discrete resistorvalue using resistors R5, R7, R25, R9, R22, or R26. The discreteresistor value is used to realize an appropriate FET gate bias voltagefor linear operation and minimum power consumption in the poweramplifier Q3. Potentiometer R6 is provided as a vernier control to setthe nominal bias current required by the particular laser diode beingused. Ferrite core inductor L2 is provided as an inductive choke toprevent RF noise that may be present on the gate bias voltage fromcontaminating the modulation signal at the input to the power amplifierQ3.

[0099] The laser driver 300 may also incorporate, as an optionalfeature, a low bandwidth signal into the broadband communicationssignal. This narrowband signal may be used, for instance, for telemetryor tracking. The narrowband signal is to be of a lower bandwidth so thatit does not interfere with the communications signal. In the embodimentof the transceiver described herein, the communications signal operateson a frequency of greater than 100 kHz, therefore, in order to avoidcontaminating the communications signal with the narrowband signal andto permit convenient filtering of the narrowband signal from thecommunications signal, the narrowband signal preferably occupies afrequency range less than 100 kHz, and more preferably less than 50 kHz.Additionally, because atmospheric scintillation primarily occupies thefrequency range below 200 Hz, the narrowband signal preferably occupiesa frequency range greater then 200 Hz to avoid corruption through theAC-coupling. Such a narrowband signal may be used by the firsttransceiver to communicate with the second transceiver the signalstrength being received. Thus, when the output power of the laser diodesrequires an increase or decrease, the transceivers may communicate sucha need and automatically compensate their respective output powers usingthe aforementioned optional digitally controlled power feature.

[0100] In FIGS. 9-10C, such a narrowband telemetry signal isincorporated through the laser bias circuitry. The narrowband modulationsignal is imposed upon the DC laser bias current, and therefore imposedupon the output of the laser driver to the laser diode. The narrowbandmodulation input to the DC bias current is a standard TTL interface intoan analog switch U5. A potentiometer R17 is used to set the desiredratio of the narrowband current to the DC bias current. Preferably, thenarrowband current is less than 20% of the DC bias current. Morepreferably, the modulation amplitude of the narrowband signal is lessthan 10% but more than 5% of the DC bias current. For example, a nominal350 mA DC bias current is preferably modulated by a narrowband signalhaving a 20 to 35 mA range. Amplifiers U3B and U3C superimpose thenarrowband modulation on the DC bias current originating at resistorR18. If no narrowband input is provided into the analog switch U5, theDC bias circuit provides only a steady DC bias current.

[0101] Baud rates ranging from 100 bps to 100 kbps may be accommodatedby the narrowband signal with the circuitry herein described. Forexample, in one implementation the narrowband signal may comprise a 9600bps Manchester-coded bit stream at 19.2 kBd, a signal which occupies aspectrum ranging from approximately 500 Hz to 20 kHz.

[0102] A second optional feature included in the laser driverillustrated in FIGS. 9-10C is monitoring circuitry that senses the DCbias current (“bias-test”) and the peak-to-peak amplitude of the lasermodulation current (“RF-test”). This monitoring circuitry, whenincluded, is preferably coupled to the aforementioned controller 250.The DC bias current is monitored through an op-amp U3D which senses thevoltage drop across two in parallel resistors R11, R19, providing a 0.5ohm impedance, in series with the laser diode drive current. The lasermodulation current amplitude is monitored by sensing the modulatedvoltage across the same two in parallel resistors R11, R19, wherein themodulated voltage is amplified by an op-amp Q4, rectified by a diodepair D7 and integrated by a capacitor C29 and a resistor R38 at theinput to op-amp U3A. The op-amp U3A generates the monitor voltage“RF-test out” that is linearly related to the laser modulation currentamplitude.

[0103] Yet another optional, but preferred, feature included in thelaser driver is a thermoelectric cooler (TEC) controller. A TEC is usedto stabilize the temperature of the laser diodes against temperaturefluctuations, thus reducing thermal stress. Reducing thermal stress tothe laser diode typically stabilizes laser wavelength over time andenables the laser diode to have a longer life. The TEC controller asdescribed herein permits highly efficient control over the TEC undercircumstances where the laser drivers are operating at maximum load,even in conditions of hot weather. In fact, operating of thethermoelectric cooler power amplifier as a controlled current source tosupply the thermoelectric cooler results in near-perfect efficiency whenmaximum cooling is required.

[0104] The TEC controller illustrated in FIGS. 9-10C operates off a 5Vsupply. A power FET U8, preferably an NDT 456P or similar device, isused as a voltage-controlled current source to supply current to theTEC. The power FET U8 and the TEC TM2 are connected across the 5Vsupply. The TEC TM2 used preferably (1) provides the required maximumcooling capacity when operated at a near-optimum current, and (2) has animpedance at this optimum operating current that results in a 5V dropacross the TEC TM2. Thus, when maximum cooling capability is required ofthe TEC TM2, the 5V supply voltage is dropped entirely across the TECTM2 and only about 0.1V is dropped across the power FET U8, resulting ingreatly enhanced efficiency under the most thermally stressingconditions. The power FET U8 is controlled through four op-amps U1A,U1B, U1C, U1D, which preferably comprise an LM2902M or similar device.

[0105] A thermistor TM1 having a 10 kohm impedance at 25° Celsius isused to sense the temperature of the laser diode. A regulated 5V supplysource is provided through a 40 kohm resistor R11 and the thermistorTM1, resulting in a nominal 100 μA current through the thermistor TM1and a voltage drop of approximately 1V across the thermistor TM1 at 25°Celsius. The thermistor is advantageously operated as part of a voltagedivider circuit. The voltage drop across the thermistor TM1 changes withtemperature and is sensed by op-amp U1A. The same 5V supply source isused to generate a 1V reference voltage using resistors R12, R34. This1V reference source is sensed by op-amp U1B. In other words, the TECcontrol circuit compares temperature sensor voltage drop from thethermistor with a reference voltage, which corresponds to a voltage thatwould result if the thermistor were operated at a desired setpointtemperature. The difference between the output voltages of op-amps U1Aand U1B is amplified by op-amp U1C. The output from op-amp U1C is theerror signal associated with the desired temperature setpoint versus theactual laser temperature. The temperature setpoint in the embodimentdepicted is 25° Celsius. Op-amp U1D and its associated resistors R45,R46 provide the error signal to the power FET U8 with the appropriategate bias resistors R43, R44 and loop gain. The control signal to thepower FET is integrated by resistor R47 and capacitor C8 to provide agentle time constant so the current applied to the TEC is not impulsive.The time constant is preferably greater than 0.5 second, and morepreferably approximately 1.0 second. Such a gentle time constant isdesirable so that stress to the TEC is minimized, thus extending thelife span of the TEC.

[0106] In a preferred embodiment utilizing TECs to cool the laserdiodes, two TECs are coupled in series to cool a single laser diode. TheTECs preferably comprise Melcor CPO 0.9-21-06 or similar devices. SuchTECs operate at nearly optimum efficiency at 1.25 A and have animpedance of 2 ohms when operated at such a current. Thus, the totalimpedance of the two in series TECs is 4 ohms. When the maximum currentof 1.25 A is drawn, approximately the full 5V supply voltage is droppedacross the TECs, resulting in less than a 0.1V drop across the power FETU8 under maximum cooling circumstances. In less demanding conditionsrequiring less cooling, the power FET U8 controls the current to theTECs by dropping some of the voltage across Vds. For example, when only50% of the maximum current is required to cool the laser diode, the TECsdraw approximately 0.63 A and drop 2.5V, with the remaining 2.5Vappearing as Vds. Such circumstances represent a worst case scenario fordissipation in the controller, which is only 1.6 W. However, this worstcase scenario occurs in cooler weather when the thermal stress on theoverall system is reduced and the overall heat load can be more readilydissipated into the surrounding environment.

[0107] The above TEC controller circuit may be modified to use a 3.3 Vsupply source instead of a 5 V supply source. A 3.3 V supply source ismore appropriate for less demanding cooling requirements. The TEC mayalso be modified to act as a both a cooling and heating element withinthe transceiver by proving a bipolar voltage supply source of +5V and−5V.

[0108] Notably, operation of a TEC as described above is not limited tocontrolling heat transfer from laser diodes. Instead, the TEC operationdescribed above could apply to most any item that needs to betemperature-controlled. Operation of a power amplifier (preferably byway of a low voltage power source) as a controlled current sourceprovides near-perfect efficiency when maximum cooling is required. Whendesigning such a cooling system, the maximum cooling requirement of theitem to be temperature controlled should be considered in light of thefact that a thermoelectric cooler has a characteristic impedance and anoptimal operating current. The desired result may be achieved byselecting the optimal operating current of the thermoelectric cooler tocorrespond with the maximum cooling requirement of the item to betemperature controlled, and further selecting the impedance of thethermoelectric cooler to drop substantially all of the supply voltagewhen the thermoelectric cooler is operated to provide the maximumcooling requirement of the item. If maximum cooling efficiency isdesired, then lower power supply voltages are preferable.

[0109] Accordingly, improved transceivers for transmission using lasersignals originating in digital format are disclosed. Further disclosedare improved laser driver circuits capable of driving lasers with highfrequencies and high currents, from either digital or analog inputsignals. Still further disclosed are methods and apparatus for operatinga thermoelectric cooler with high efficiency, either with or without anaccompanying laser diode. While embodiments and applications of thisinvention have been shown and described, it would be apparent to thoseskilled in the art that many more modifications are possible withoutdeparting from the inventive concepts herein. The invention, therefore,is not to be restricted except in the spirit of the appended claims.

What is claimed is:
 1. A communication link for providing two-waycommunication through free space, the link including a first transceiverand a second transceiver, wherein at least one transceiver comprises: aninput signal interface for receiving a digital signal; a splitter incommunication with the input to split the digital signal into aplurality of approximately equal laser data signals; a plurality oflasers displaced from one another and facing in parallel directions,each of the lasers being in communication with the splitter; a pluralityof laser drivers, each laser driver being coupled to one of the lasersand to the splitter, wherein the laser drivers receive the laser datasignals and provide amplified laser data signals to the lasers at highpower and high frequency.
 2. The communication link of claim 1, whereinthe input signal is characterized by a data rate of at least 10Mbits/second, and each laser is supplied with a nominal current of atleast 100 mA.
 3. The communication link of claim 1, wherein said atleast one transceiver further comprises an input signal regenerator incommunication with the one or more digital input signals.
 4. Thecommunication link of claim 3, wherein the input signal regeneratorincludes a first clock and data recovery circuit which is switchablebetween one of a plurality of clock frequencies.
 5. The communicationlink of claim 1, wherein each of the plurality of lasers includes alaser diode coupled to the laser driver and receiving the amplifiedlaser data signals.
 6. The communication link of claim 5, wherein eachof the plurality of lasers further includes a lens for receiving andcollimating the laser diode output into an optical beam.
 7. Thecommunication link of claim 6, wherein the beamwidth of the optical beamis adjustable.
 8. The communication link of claim 7, wherein thebeamwidth of the optical beam is adjustable between from 0.3 mrad toapproximately 3.5 mrad.
 9. The communication link of claim 6, whereinthe beamwidth of the optical beam is less than 3.5 mrad.
 10. Thecommunication link of claim 5, wherein the laser driver includes amodulation signal amplifier coupled with the splitter and a DC biascircuit coupled between the modulation signal amplifier and the laserdiode.
 11. The communication link of claim 10, wherein the laser driverfurther includes a sampling photodiode, wherein the modulation signalamplifier is coupled with the sampling photodiode and responsive to theoutput of the sampling photodiode.
 12. The communication link of claim11, wherein the laser diode emits a laser beam, and the samplingphotodiode monitors the power of the laser beam.
 13. The communicationlink of claim 5, wherein said at least one transceiver further comprisesa thermoelectric cooler in thermal communication with the laser diode.14. The communication link of claim 5, wherein the laser driver operatesat a current of between approximately 100 milliAmperes and 1500milliAmperes.
 15. The communication link of claim 5, wherein the laserdiode generates an average power of at least 80 milliwatts.
 16. Thecommunication link of claim 1, wherein the digital signal a comprisespacket-based communication signal in accordance with a data transmissionprotocol.
 17. The communication link of claim 16, wherein the datatransmission protocol complies with a protocol selected from the groupconsisting of TCP/IP, IPX, Fast Ethernet, SONET, and ATM.
 18. Thecommunication link of claim 1, wherein the transceiver is capable ofoperating on at least one layer selected from the group consisting ofSTS-3, STS-12, OC-3, and OC-12.
 19. The communication link of claim 1wherein said at least one transceiver further comprises: an aperture; areflector in line with the aperture; a photodiode at the focal point ofthe reflector; and an output from the photodiode.
 20. The communicationlink of claim 19, wherein the reflector has an f-number of about 0.6721. The communication link of claim 19, wherein the reflector is aMangin mirror.
 22. The communication link of claim 19, wherein thereflector is a mirror having a general conic, aspheric, or parabolicoptical surface that is coupled with one or more corrector lenses. 23.The communication link of claim 19, wherein said at least one lasertransceiver further comprises a stray light baffle disposed across theprimary aperture.
 24. The communication link of claim 19, wherein saidat least one transceiver further comprises: a preamplifier coupled withthe photodiode; an output signal regenerator coupled with thepreamplifier; and an output signal interface coupled with the outputsignal regenerator.
 25. The communication link of claim 24, wherein theoutput signal regenerator includes a second clock and data recoverycircuit which is switchable between one of a plurality of clockfrequencies.
 26. The communication link of claim 19, wherein said atleast one transceiver further comprises a background rejection filternear the focal point of the reflector.
 27. The communication link ofclaim 26, wherein the background rejection filter is a bandpass filter.28. The communication link of claim 26, wherein the background rejectionfilter is an optical interference filter and has a nominal centerwavelength of approximately 1550 nanometers.
 29. The communication linkof claim 26, wherein said background rejection filter is a long wavepass filter having a threshold passage wavelength, said at least onetransceiver further comprising a detector having a predictableresponsivity roll-off at a wavelength above the threshold passagewavelength of the long wave pass filter.
 30. The communication link ofclaim 19, further comprising a radio frequency backup transceiver. 31.The communication link of claim 30, wherein said at least onetransceiver includes monitoring circuitry for monitoring signal strengthor transceiver status.
 32. The communication link of claim 31, whereinthe backup transceiver is activated upon detecting impairment of thelaser transceiver, and the backup transceiver is deactivated upondetecting non-impairment of the laser transceiver.
 33. The communicationlink of claim 30, wherein said at least one laser transceiver operateswith the backup transceiver in overflow mode.
 34. The communication linkof claim 1, wherein said at least one laser transceiver furthercomprises a protective enclosure.
 35. The communication link of claim34, wherein said at least one laser transceiver further comprises anenvironmental control system for maintaining a desired temperature andhumidity within the enclosure.
 36. The communication link of claim 1,wherein said at least one transceiver further includes a multiplexer tocombine multiple signal inputs and a de-multiplexer to segregatemultiple signal outputs.
 37. A digital signal transceiver comprising: aninput signal interface for receiving a broadband digital signal; a firstregenerator coupled with the input signal interface; a splitter coupledwith the regenerator to split the digital signal into one or more laserdata signals; a high power, high frequency laser driver coupled with thesplitter to condition the laser data signals; and a plurality of laserscoupled with the laser driver to receive the laser data signals, thelasers being laterally displaced from one another and facing in paralleldirections.
 38. The transceiver of claim 56, wherein the firstregenerator includes a first clock and data recovery circuit which isswitchable between one of a plurality of clock frequencies.
 39. Thetransceiver of claim 37, wherein each of the plurality of lasersincludes a laser diode coupled to the laser driver and receiving theconditioned laser data signals.
 40. The transceiver of claim 39, whereineach of the plurality of lasers further includes a lens for receivingand collimating the laser diode output into a beam having a beamwidth of3.5 mrad or less.
 41. The transceiver of claim 39, wherein the laserdriver includes a modulation signal amplifier coupled with the splitterand a DC bias circuit coupled between the modulation signal amplifierand the laser diode.
 42. The transceiver of claim 41, wherein the laserdriver further includes a sampling photodiode, wherein the modulationsignal amplifier is coupled with the sampling photodiode and responsiveto the output of the sampling photodiode.
 43. The transceiver of claim42, wherein the laser diode emits a laser beam, and the samplingphotodiode monitors the power of the laser beam.
 44. The transceiver ofclaim 39, wherein the laser driver further includes a thermoelectriccooler adjacent to the laser diode.
 45. The transceiver of claim 39,wherein the laser diode operates at a current of approximately between100 milliAmperes and 1500 milliAmperes.
 46. The transceiver of claim 39,wherein the laser diode generates an average power of at least 80milliwatts.
 47. The transceiver of claim 37, wherein the digital signalcomprises packet-based communication signals in accordance with at leastone data transmission protocol.
 48. The transceiver of claim 47, whereinthe at least one data transmission protocol complies with a protocolselected from the group consisting of TCP/IP, IPX, Fast Ethernet, SONET,and ATM.
 49. The transceiver of claim 47, wherein the transceiveroperates on at least one layer selected from the group consisting ofSTS-3, STS-12, OC-3, and OC-12.
 50. The transceiver of claim 37, furthercomprising: an aperture; a reflector in line with the aperture; aphotodiode at the focal point of the reflector; a preamplifier coupledwith the photodiode; a second regenerator coupled with the preamplifier;and an output signal interface coupled with the second regenerator. 51.The transceiver of claim 50, wherein the reflector is selected from thegroup consisting of a Mangin mirror, a parabolic reflector coupled witha corrector lens, and a mirror having a general conic or asphericoptical surface and coupled with at least one corrector lens.
 52. Thetransceiver of claim 50, wherein the second regenerator includes asecond clock and data recovery circuit which is switchable between oneof a plurality of clock frequencies.
 53. The transceiver of claim 50further comprising a background rejection filter disposed adjacent tothe focal point of the reflector.
 54. The transceiver of claim 53,wherein the background rejection filter is an optical hemisphericalinterference filter having a nominal center wavelength of approximately1550 nanometers.
 55. The transceiver of claim 50, wherein the reflectorhas an f-number of about 0.67.
 56. An apparatus for efficiently drivinga laser diode, the apparatus comprising: a signal source providing aninput signal; a laser diode having a characteristic impedance; and apower amplifier with a low output impedance suited to drive the laserdiode; wherein the power amplifier is operated as a voltage-controlledcurrent driver for the laser diode.
 57. The apparatus of claim 56,further comprising a voltage amplification stage between the signalsource and the power amplifier.
 58. The apparatus of claim 57, whereinthe voltage amplification stage includes a non-linear limitingamplifier.
 59. The apparatus of claim 56, wherein the power amplifierincludes a broadband RF power field effect transistor.
 60. The apparatusof claim 59, wherein the power field effect transistor is selected fromthe group consisting of MOSFET, silicon FET, and GaAs FET.
 61. Theapparatus of claim 59, wherein the broadband RF power field effecttransistor is capable of operating at a minimum frequency of 1 MHz orless.
 62. The apparatus of claim 56, wherein the broadband RF powerfield effect transistor is operated with a low supply voltage.
 63. Theapparatus of claim 56, wherein the supply voltage of the power amplifieris approximately equal to or less than 12 volts.
 64. The apparatus ofclaim 56, wherein the supply voltage of the power amplifier isapproximately 5 volts.
 65. The apparatus of claim 56, wherein the powertransistor provides output current of at least 100 mA to the laserdiode.
 66. The apparatus of claim 65, wherein the input signal ischaracterized by a data rate of at least 10 Mbits/second.
 67. Theapparatus of claim 65, wherein the input signal is characterized by adata rate of at least OC-3 bandwidth.
 68. The apparatus of claim 56,wherein the power transistor provides output current of at least 200 mAto the laser diode.
 69. The apparatus of claim 56, wherein the inputsignal is a signal selected from a group of protocols consisting of:TCP/IP, IPX, Fast Ethernet, SONET, and ATM
 70. The apparatus of claim56, wherein the laser diode has a characteristic dynamic impedance ofbetween approximately 2 and 5 ohms.
 71. The apparatus of claim 56,further comprising a thermoelectric cooler in thermal communication withthe laser diode.
 72. The apparatus of claim 56, wherein the laser diodeis stabilized against temperature fluctuations.
 73. The apparatus ofclaim 56, wherein the power amplifier is stabilized against supplyvoltage fluctuations.
 74. The apparatus of claim 73, further comprisinga zener diode for stabilizing power supply voltage against voltagefluctuations.
 75. The apparatus of claim 56, further comprising anattenuator between the signal source and the power amplifier.
 76. Theapparatus of claim 75, wherein the attenuator is adjustable and is usedto control the amplitude of the input signal to the power amplifier. 77.The apparatus of claim 56, further comprising: a temperature sensor forsensing temperature of the laser diode, a thermoelectric cooler inthermal communication with the laser diode, and a thermoelectric coolerpower amplifier, wherein the thermoelectric cooler power amplifier isoperated as a controlled current source to supply current to thethermoelectric cooler at near-perfect efficiency when maximum cooling isrequired.
 78. The apparatus of claim 77, wherein the temperature sensoris a thermistor.
 79. The apparatus of claim 78, wherein the voltage dropacross the thermistor at a given temperature is compared to a referencevoltage corresponding to the thermistor voltage when it is operated at adesired setpoint temperature.
 80. The apparatus of claim 77, wherein thepower amplifier is a power FET.